Silence, believers: a new deity has swept into the Lost in Showbiz pantheon and you will kneel before him. Taking his place next to the likes of Trudie Styler, goddess of flight and couriered truffles, comes a god of awesome logical prowess and mighty self-regard. In hieroglyphic form he is represented as a giant speculum, but in the demotic he is written "Jeremy Irons".

At present, you would probably react to that with the words, "Ah, the actor," – in which case, you are excruciatingly out of date. Know this: the correct incantatory response to hearing the name Jeremy Irons is "Ah, the newly minted green campaigner, whom we must herald as one of the age's most credible ecological thinkers, even though he owns loads of cars, six large houses, and one pink castle."

But we race ahead of ourselves.

Certainly, Jeremy has hitherto been best known as an actor, with the speculum a reference to what he has judged his finest work, the chick flick Dead Ringers. Face it, nothing says Lambrini and popcorn like a synopsis that is basically "Jeremy Irons plays identical twin gynaecologists, one of whom descends into narcotic delusions and designs his own . . . special instruments . . . to deal with the mutant female reproductive systems that come under his care". And still blokes shudder at Deliverance. Man up, men!

Now where were we? Oh yes. Something awfully important has happened. Jeremy used the occasion of a Sunday Times interview last weekend to announce that he is going to make a documentary about sustainability and waste disposal, in the vein of Michael Moore, though "not as silly". The first sadness is that this isn't a straight job swap – from what Lost in Showbiz can establish, Michael Moore has no plans to make a movie in which he plays a middle-aged urban professional going through some form of crisis which will manifest itself in a singularly messed-up sexual obsession. So anyone hoping for a scene in which the Fahrenheit 9/11 star grinds away on top of some nubile young thing on an operating table or whatever will be heartbreakingly disappointed.

The second and somewhat greater sadness is Jeremy's decision to pose as the under-read heir to Thomas Malthus, explaining that, "One always returns to the fact that there are just too many of us, the population continues to rise and it's unsustainable." Would the Damage and Die Hard 3 star care to explain how it will all end? "I suspect," obliges Jeremy, "there'll be a very big outbreak of something because the world always takes care of itself." It's nature's way, innit? Trivia buffs may care to know that Jeremy reckons the big outbreak of something could be either war or disease, but "probably disease", which will allow Mother Earth to halve the population.

How this all fits in with earlier glimpses of Jeremy's philosophy is difficult to say. A few years ago, he was explaining that, "I believe in coincidence: if you're destined to become rich, you will – if not, too bad for you."

Still, this new direction is hardly unexpected from a performer who could never be accused of not taking himself seriously enough. There are actors such as Michael Caine who occasionally do the most frightful crap, but are charming enough to be funny about it. "I have never seen it," explained Sir Michael of his outing in Jaws IV: the Revenge. "However, I have seen the house that it built, and it is terrific." So as I say, there are those actors. And then there are the likes of Jeremy, who once pronounced, "Once in a while, you have to risk a bit." The role of which he was speaking was that of an evil wizard in Dungeons & Dragons, a hilariously bad movie based on the role-playing game. Or as he put it: "Like Alec Guinness in Star Wars, I had to give the project some gravitas." Mm.

Yet I note that some years later, Jeremy did begin justifying himself with an inferior version of Caine's line. "Are you kidding?" he inquired of those wondering why he'd taken the role. "I'd just bought a castle, I had to pay for it somehow."

And therein, you can't help thinking, lies the problem with his philosophy. Some people make bad decisions out of naked self-interest, which is all very well – but whether they are the right people to be fronting campaigns telling others how to live is quite another matter. As we have previously observed in the case of Trudie, it's notable how often celebrity crusaders fail to see any sort of a continuum between their own inability to make such minuscule lifestyle changes as ceasing their dependence on private jet travel, and governments' inability to face up to the need for systemic changes.

"How many clothes do people need?" wonders Jeremy, who I think I mentioned has just the six houses and a castle in a flattering shade of pink. "People must drop their standards of living so the wealth can be shared about," he explains. "There's a long way to go."